The influence of wheel speed and melt temperature on the surface characteristics of slag fibers were examined in mineral fibers synthesized from the steel-making slag, using a modified melt extraction method, i.e. melt spinning. The synthesized fibers were characterized by optical microscope and scanning electron microscopy. It was found that the wheel speed of 700 rpm generated better quality of mineral fibers in terms of diameter and length. This was attributed to the relative extent of contact width between the flowing melt and the rotating wheel. The thickness of the slag fibers were also decreased with increasing the slag melt temperature due mainly to significant decrease in the viscosity of the slag melt. In addition, the lower melt temperature caused an increase in number of shots plus the mineral fibers.